Urban development necessarily covers a substantial proportion of the surface of the ground with buildings, adjacent parking areas and interconnecting roadways. Subsequent to a rainstorm the consequences of this development are manifested in higher peak runoff volumes, increased turbidity and pollution in runoff water, decreased water infiltration into the ground, increased pollution in receiving water bodies, flooding problems within waterways and even changes in the subsurface water level.
One study estimated that while 40% of rural rainfall was absorbed into the ground prior to urban development, as little as 10% of urban rainfall is now so absorbed.
Accordingly, the conveyance and disposition of increased and more polluted volumes of storm sewage is one of the major considerations that both a developer and an urban government must grapple with when contemplating urban development.
Storm sewage systems in the past have employed drainage wells to put storm water back into the ground. An example of this is shown in Austrian Patent No. AT-B-347 864. A filter is shown in this patent to avoid polluting the ground water. However, a long, vertical, tubular filter is disclosed which would easily plug-up, and in any event, the storm water would not be filtered uniformly because of the difference in static pressure or head from the bottom to the top of the filter.
German patent No. DE-U-8 236 344 shows a filter located in a well, but most of the filter is submerged and storm water is free to run over the top and bypass the filter. As a result, the filter would not be very effective.